Y=A+B. This is the equation of OR gate. We require 3 NAND gates to create  OR gate.  We can also write

After 1^st NAND operation
Y = (A AND B)' 
Y=  A'  + B'  (Demorgan's Law)
After 2^nd NAND operation
Y= ( A'  + B')'
Y=  A . B  (Demorgan's Law)
After 3^rd  NAND operation
Y= ( A . B )'
Y=  A' +  B '  (Demorgan's Law)

So we need 3 NAND gates.